Metal foil, such as copper foil, has become widely used in a wide variety of electronic and electrical component technologies. A separate field of technology has developed for the production of metal foils useful in such industries to achieve properties useful for various applications. Commercially, the primary means by which desirable properties are added to metal foil is by electrodeposition of metals from metallic ion-containing baths. Such processes have been used to produce a matte surface having microscopic dendritic (i.e., tree-like or nodular) structures to aid in adhering the foil surface to other materials. Electrodeposition has also been used to apply certain metals as a thermal barrier, an elevated temperature metal diffusion barrier, an oxidation barrier, a chemical corrosion barrier, and/or provide certain electrical properties such as electric current resistance.
After it has been treated to achieve the aforementioned properties, metal foils, such as copper, tin and nickel foils, are particularly well suited for use in various electronic and electrical components. Of particular interest are printed circuit boards and PCB components, especially multilayer PCB laminates, solid state switches and the like which have been developed to meet the demand for miniaturization of electronic components and the need for PCBs having a high density of electrical connections and circuitry. The technologies for the production of copper foils by electrodeposition from electroplating baths and/or processing the metal in a roll mill are well known in the art.
A typical example of production of a metal foil for electronic applications is the production of copper foil by electrodeposition processes. Such processes generally involve the use of an electroforming cell (EFC) consisting of an anode and a cathode, an electrolyte bath solution, generally containing copper sulfate and sulfuric acid, and a source of current at a suitable potential. When voltage is applied between the anode and cathode, copper deposits on the cathode surface.
The copper foil making process begins by forming the electrolyte solution, generally by dissolving (or digesting) a metallic copper feed stock in sulfuric acid. After the copper is dissolved the solution is subjected to an intensive purification process to ensure that the electrodeposited foil contains no disruptions and/or discontinuities. Various agents for controlling the properties may be added to the solution.
The solution is pumped into the EFC and when voltage is applied between the anode and cathode, electrodeposition of copper occurs at the cathode. Typically, the process involves the use of rotatable cylindrical cathodes (drums) that may be of various diameters and widths. The electrodeposited foil is then removed from the cylindrical cathode as a continuous web as the cathode rotates. The anodes typically are configured to conform to the shape of the cathode so that the separation or gap therebetween is constant. This is desirable in order to produce a foil having a consistent thickness across the web. Copper foils prepared using such conventional electrodeposition methodology have a smooth shiny (drum) side and a rough or matte (copper deposit growth front) side.
Conductive foils for PCB applications and other electronic devices are conventionally treated, at least on the matte side, for enhanced bonding and peel strength between the matte side and the laminate. Typically the foil treatment involves treatment with a bonding material to increase surface area and thus enhance bonding and increase peel strength. The foil may also be treated to provide a thermal barrier, which may be brass, to prevent peel strength from decreasing with temperature. Finally, the foil may be treated with a stabilizer to prevent oxidation of the foil. These treatments are well known in the art.
Recently, there has been a need for conductive foils exhibiting improved peel strength and capable of enduring thermomechanical stress due to increasing reliance upon electronic components in harsh environments. Electronically controlled devices are becoming more common under the hoods of vehicles, such as cars, trucks, and heavy equipment, for microprocessor control of combustion conditions and in equipment used in industrial environments which subject the circuits to high temperatures and/or mechanical stress such as electronic chemical or metallurgical process control equipment, robotic equipment, etc. Stress may also be induced due to harsh geographical environments.
Attempts to increase peel strength by deposition of a high profile dendritic metal electrodeposit on metallic foil have been limited due to a formation of what is known as a "dusty" or "very dusty" deposit. The "dusty" nature of electrodeposited layer is due to the formation of large nodules having insufficient dendritic support, resulting in large numbers of nodules becoming easily detached from the foil to form the loose "dusty" surface. Dusty high profile electrodeposits have low or inadequate peel strengths due to this weakness in the dendritic anchoring of the foil to another substrate, such as laminate.
Not only is peel strength effected, but another problem arises, known as "treatment transfer" in which the electrodeposited metal separates from the foil as metal particles and becomes imbedded in the laminating material. This is undesirable since such particles could cause unintended flows of electrons (i.e., short circuits) between what would otherwise be insulated layers of conductive material in the multilayer circuit board laminates.
The present invention provides a procedure for obtaining a metallic foil having a nodular high profile dendritic metal deposit which does not suffer from the aforementioned drawbacks and provides additional desirable properties.